Telephone system



March 29, 1949. B. c. GROH 2,465,372

TELEPHONE SYSTEM Filed Aug. 10, 1945 INVENTOR. BERNARD c. GROH BY WaraEd Patented Mar. 29, 1949 I a. S

ATENT OFFICE (Granted under the act of March a, 1883, as

amended April so, 1928; 370 0. G. 757) 2 Claims.

The invention described herein may be manufactured and used by or forthe Government for governmental purposes, without the payment to me ofany royalty thereon.

The invention relates to telephone systems.

Telephone systems commonly employ a switchboard signalling devicewhereby the switchboard operator may be made aware that a signal hasbeen impressed on a given line. In an ideal system, the signal devicewould be responsive to signals of a predetermined character and would benonresponsive to other signals. Heretofore this desired performance hasnot been obtained, since momentary surges of electrical energy fromsources external to the telephone circuit have been able to cause theswitchboard signal device to operate, thus impressing a false indicationof an incoming signal on the system.

It is therefore an object of the invention to provide a telephone systemof a novel character whereby switchboard signal devices may b triggeredonly when a signal of a predetermined character is impressed on theline.

In a preferred embodiment of the invention, means are positioned in theelectrical circuit between the source of signal energy and the signalresponsive switchboard devic whereby the signal responsive device willbe triggered only by signals of a predetermined character, the saidmeans comprising a discriminator circuit of a novel character.

A better understanding of the invention will be obtained from thefollowing description of the preferred embodiment of the invention, readin connection with the accompanying drawing, in which the figure is aschematic circuit diagram of the said preferred embodiment.

Referring to the figure, a discriminator circuit l0, comprisingnon-inductive resistance arms l2 and trigger arm M, is electricallypositioned between a signal generator l6 such as a field telephone, anda four element neon signal device I8.

The discriminator circuit I is in the form of a Wheatstone bridge. Thetrigger arm l4 comprises a thermistor. Thermistors are well known in theart and are characterized by negative resistanc characteris ic. Morespecifically, the resistance of a thermistor decreases after a currenthas passed therethrough for an appreciable amount of time. This negativeresistance characteristic is caused by the thermal effect of the currentpassing therethrcugh. It is therefore a result, that if a very shortsurge of current passes through the element there will be no appreciableresistance change, whereas if a current having a duration suiiicient toheat the element passes therethrough, the heating caused thereby will besufficient to cause the resistance of the said element to drop sharply.For example, a current having a duration of of a second will cause theresistance of a standard thermistor to drop from 50,000 ohms toapproximately 1500 ohms. This characteristic of a thermistor is utilizedin this invention to discriminate against momentary surges of current ina telephone line that heretofore have been sufficient to operate aswitchboard signal device and impress thereon a false indication of anincoming signal.

The field telephone l6 provides a signal hav-- ing a durationsufliciently long to operate the thermistor. This signal transmittedover a transmission line 20, is impressed upon the bridge III atconjugate points a, a The bridge, being normally balanced, does notinitially present a potential at points 1), b across which points theneon signal device I8 is connected. However, after the signal currenthas been impressed upon the bridge circuit ID for a fraction of asecond, the resistance of the thermistor l4 drops sharply, causing apotential to appear across conjugate points 11, 12 which potential,being impressed upon the elements 22 of neon signal tube I8, issufficient to trigger the tube.

Assume, however, that a momentary surge of electrical energy occurs inthe transmission line 20 somewhere between the discriminator circuit i0and the field telephone Hi. This surge would be impressed upon points a,a of the discriminator circuit l0. However, since the duration of asurge is very short, the heating effect of the surge would not besufiicient to cause the resistance of the thermistor Hi to drop. Therewould thus be developed no potential across point b, 11 and therefore nosignal would be indicated on signal device I8.

I have thus provided a means whereby a switchboard signal device may beinsulated against momentary surges of energy in a telephone system, thesaid means being adapted to respond to desired signals.

Although the operation of the invention has been illustrated by a deviceincorporating a thermistor, it is obvious that any element having adelayed response, whereby the bridge may be unbalanced by an impressedsignal, may be substituted for the thermistor.

The neon signal device 18 is. provided with a separate pair ofelectrodes 24 whereby the signal device will remain operative untilextinguished by the switchboard operator. The electrodes 24 areconnected to battery 26 through key 28., The potential of battery 26 ischosen at a value below the firing voltage of tube l8 but above theextinction voltage thereof. This sustaining circuit is normally closedthrough key 28, the voltage developed therein being insufficient to firethe tube H; but suflicient to sustain the tube once it has been fired.Thus if the tube of the device is fired by an incoming signal of apredetermined character operating through discriminator circuit Hi, thesaid tube will remain fired until the key 28 is opened by theswitchboard operator. This feature is very important in that itmaintains the flash on the board until removed by the operator, thuseliminating the possibility of an incoming signal being overlooked.

It is obvious that many modifications and changes may be made within thescope of the invention, the invention being fully defined in theattached claims.

What is claimed is:

1. The combination, in a telephone system, of signal generating meansfor signalling an attendant, said signals having a duration in excess ofa predetermined value, a bridge network having a pair of conjugatepoints connected across said source, a resistive element having apredetermined negative characteristic integral in only one of the armsof said network whereby the said network will be rendered unbalancedupon receipt of a signal of the aforementioned characteristic, and meansconnected across the other pair of conjugate points of said networkresponsive to and triggerable by the said unbalance, the said responsivemeans comprising a four element neon lamp. said elements comprising twopairs of electrodes, one pair of said electrodes being connected to thesecond named pair of conjugate points and the other pair of saidelectrodes being con- 4 nected to a source of voltage having a valuebelow the firing point of said lamp and above the extinction valuethereof.

2. In a telephone system, the combination of signal generating means forsignalling an attendant, said signals having a duration in excess oi! apredetermined value, a bridge network comprising three arms ofresistance elements having a linear resistance characteristic, and afourth arm including a thermistor, one pair of conjugate points of saidbridge network being connected across said generating means, a fourelement neon lamp, two of said elements being connected to the otherpair of conjugate points of said bridge network and being triggered bythe unbalance voltage of said bridge upon receipt of a signal of theaforementioned characteristic, the other set of said elements beingconnected to a source of voltage having a value below the firing pointof said lamp and above the extinction value thereof, whereby the saidlamp, upon being triggered, will continue to glow upon cessation of saidsignal until the connection to said source of voltage is broken.

BERNARD C. GROH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 854,121 Storer May 21, 19071,276,589 Steinmetz Aug. 20, 1918 1,893 638 Schmierer Jan. 10, 19332.096,865 Swart Oct. 26, 1937 2,103,439 Swart Dec. 28, 1937 2,377,506McWhirter June 5, 1945

